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Snow Avalanche Problems
This report by the Committee on Ground Failure Hazards Mitigation Research ad-
dresses the problems and mitigation issues concerning snow avalanche hazards in the United
States. Other reports by the committee have considered problems due to landslides (Na-
tional Research Council, 1985) and ground subsidence (National Research Council, in
press). The present report is the first publication on snow avalanches by any National Re-
search Council committee; therefore, it is essential to include both a general and historical
perspective in order to provide sufficient background for discussion of current problems.
This information is not available elsewhere through any single published source.
The purpose of the report is to provide national, regional, and local governments;
government agencies; and private decision makers with an overview of the snow avalanche
situation in the United States and to outline steps that can be taken to minimize domestic
avalanche problems. Four major points are emphasized:
I. Support for avalanche programs has diminished alarmingly at a time when increas-
ing numbers of people are using mountain areas for recreation and commercial and other
types of development are increasing in formerly remote areas.
2. The incidence of avalanche accidents is increasing and is expected to continue to
increase in the future.
3. There is a lack of nationwide coordination, accepted standards, and effective infor-
mation flow among those involved in avalanche mitigation.
4. There are no standardized procedures for avalanche control and equipment testing.
Control techniques and equipment that use explosives have specific hazards and problems
that must be addressed.
Snow avalanches have caused natural disasters as long as mountainous areas have been
inhabited. They are a common occurrence in mountainous terrain throughout the world,
wherever snow is deposited on slopes steeper than about 20 to 30 degrees. In the United
States, where avalanches are the most frequent form of lethal mass movement, avalanche
hazard exists from the lower-elevation coastal mountain ranges to the higher mountains of
the continental interior.
s
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By definition a snow avalanche is simply snow moving rapidly down sloping terrain.
A moving avalanche may also contain soil, rock, vegetation, or water, but by definition
the initial failure that triggers an avalanche occurs within the snowpack or at the interface
between snow and subjacent terrain. Avalanches range from a harmless trickle of loose
snow descending to a new angle of repose to a huge and devastating mass of snow moving
at high speed down a long steep slope, with enough energy to destroy everything in its path.
It is important to know that, unlike other ground-failure hazards such as rockslides, which
once released are spent, snow avalanches automatically "reload" with each snowfall and can
"fire" several times in a given year.
Small avalanches or stuffs run in uncounted numbers each winter, while larger ava-
lanches, which may encompass slopes several kilometers wide and include millions of tons
of snow, release infrequently but have the potential to inflict the greatest destruction.
Avalanches of moderate size can damage structures and have the ability to bury, injure,
and kill neonie. In the United States anoroximatelv 10.000 avalanches are reported each
winter, with an estimated 10 to 100 times that number occurring unobserved or unreported
(Armstrong and Williams, 1986~.
Terrain and weather patterns combine to determine the frequency of avalanche events.
Large frequent snowstorms in combination with steep slopes will produce a high number
of avalanches during a given winter season. Under ordinary circumstances, avalanches tend
to run in the same location and down the same paths year after year, with the danger
zones often becoming well known. However, exceptional weather conditions can produce
avalanches that overrun their normal path boundaries or even create new paths where none
existed for centuries (Fitzharris, 1981), as illustrated by the destruction in Switzerland of
a 573-year-old stone building in 1957 (FriedI, 1974~. Unusually high snowfall can provide
short-lived but great hazard, in which even historically stable slopes may become dangerous
(Figure I).
A factor in most avalanche releases is the presence of structural weaknesses, often
induced by internal changes in snow cover. Hence, a large overburden of snow alone may
not result in avalanching if it is internally strong and anchored to the layer below, but
a shallow snow layer can slide from a mountainside if the snow is poorly bonded to the
underlying material. Snow avalanches represent a complex problem in mechanical stability;
thus, attempts to provide a better understanding of the phenomenon have focused primarily
on the physical processes taking place within the constantly changing winter snow cover and
the dependence of those processes on temperature and other meteorological factors.
A hazard arises whenever property or human activity lies in the path of a potential
avalanche. Snow avalanche hazard has been familiar to inhabitants of the European Alps
and Scandinavia for many centuries, but it is a more recent problem in the United States.
During the active period of gold and silver mining from IS80 to 1920, approximately 400
people were killed by avalanches in Colorado, many trapped within structures. More recently
the primary hazard has been to individuals engaged in recreation activities, with deaths and
injuries frequently occurring at some distance from developed facilities. Such events have
the potential to affect the local economy of many mountain regions and to exert a significant
effect on federally managed lands.
U.S. citizens may also be endangered by avalanche hazards abroad. Those exposed
to risk include not only Alpine recreationists (Vile, 1987) but also military personnel, as
illustrated by the 1986 NATO exercise in Norway, during which 31 men were struck by a
naturally released avalanche; 16 were killed and Il injured (Kristensen, 1986~. In a distinct
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FIGURE 1 (a) An "attractive" potential development site in a century-old lodgepole pine forest on a fan
beneath Deadman Gulch, Colorado Front Range, 1976. Small avalanches over previous several decades had
been contained by adjacent gullies. (b) The same area in May 1984, showing the eject of a "100-year" dry
snow avalanche. This avalanche far exceeded the boundaries of previously recorded events and destroyed
many acres of the pine forest that had colonized in the runout zone for over a century. These photographs
provide valuable before-and-after documentation of the "design avalanche," the event magnitude that should
be considered in land-use planning and design of exposed facilities. Because most avalanche paths have not
recently produced an event of design magnitude, many planners and others tend to ignore or underestimate
the potential avalanche threat. (Courtesy of A. Mears and Paula J. Lehr)
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category are the military catastrophes of the Tyro! in World War I, where estimates of
avalanche-caused fatalities ranged from 40,000 to 80,000 (Fraser, 1966~.
Avalanche danger is alleviated in three fundamental ways: by modifying the terrain,
by modifying the snow cover, and by motiving human behavior. A number of engineering
techniques have been used to divert or deflect moving snow from facilities; other techniques
are used to prevent destructive avalanches from releasing. Reforestation provides a natural
form of protection, but avalanche risk may substantially increase in the near future due to
forests dying or deteriorating as a result of air pollution.
The most common technique for reducing avalanche hazard is to artificially release
potential avalanches at a selected safe time. This practice inhibits the formation of large
avalanches by producing more frequent smaller ones. While this method may not provide
as high a degree of protection as some terrain-modification techniques, it is less expensive
in the short term; the technique is commonly used at ski areas and along highways anal
railroads. Avalanches are usually released by explosive charges, detonated on or near the
snow surface close to the expected fracture point. Such charges are placed by hand or
delivered to the slope using some form of artillery or mechanical conveyance.
Because avalanches can affect winter vacationers, widespread public education about
avalanches is of particular importance. Instruction on how to evaluate and avoid avalanche-
prone terrain and on rescue techniques is important for reducing hazards to downhill and
cross-country skiers and snowmobilers. The highly mobile nature of these activities makes
control with structures and explosives difficult. Centralized avalanche information and
forecast centers such as those located in Colorado, Utah, Washington, and some other areas
are an essential ingredient in avalanche education. In some cases land-use management and
zoning can be used to protect the public in avalanche-threatened areas.
Yet despite the increasing hazards posed by snow avalanches to mountain residents
and tourists in the United States, there is no coordinated national program for avalanche
mitigation. There is no recognized national leadership, no systematic means to improve
understanding of avalanche processes or to improve mitigation procedures, and no adequate
and comprehensive mechanism for information transfer and exchange.
Representative terms from entire chapter:
snow avalanches
